Preheat oven to 350°F. Line a muffin tin with paper liners. In a medium saucepan, bring the Guinness and butter to a boil. Reduce heat and simmer until the butter is melted. Add baking mix and stir to combine. Crack the eggs into a small bowl and whisk to combine. Stir eggs into Guinness mixture. Fill paper liners 2/3 full. Bake until a toothpick inserted into the center of a cupcake comes out clean, about 25 minutes. Cool before frosting.

For Frosting

With an electric mixer, combine the mascarpone, butter, and coffee. Beat in 1-1/2 cups powdered sugar. Adjust sweetness to taste with additional powdered sugar, if desired. Frost the cooled cupcakes.